1. Field of the Invention
The present invention relates to an onboard battery monitoring apparatus which monitors the charged state of a battery installed on a vehicle such as a motor vehicle and a method for correcting an offset value of a current sensor which is used in the monitoring apparatus, the apparatus and method being intended to increase the measuring accuracy for charging and discharge currents of onboard batteries.
2. Description of the Related Art
In motor vehicles produced in recent years, various types of electronic devices are installed, and the fuel injection amount of an engine is also controlled by a computer. In these circumstances, in case the amount of residual charged current in an onboard battery which supplies power to those electronic devices becomes short, a serious problem with the operation of a vehicle occurs. On the other hand, in order to realize the reduction in load placed on the engine so as to increase the fuel economy of the vehicle, it is required to reduce wasteful power generation by an alternator which charges the onboard battery.
In order to control the power generation accurately, it is necessary to be aware of an accurate charged state of the battery, and to this end, an onboard battery monitoring apparatus is provided which detects a discharge current which flows out of the battery (a discharge amount) and a charging current which flows into the battery (a charging amount) by a current sensor and estimates the charged state of the battery from an integrated value from the discharge amount and the charging amount so detected.
The current sensor is made up of, for example, a ring core having a gap, a lead wire which passes through a ring of the ring core and a Hall element inserted in the gap of the ring core, so that a pass current which flows through the lead wire is measured by metering a magnetic flux density that is produced by the pass current.
A measured value that is measured by the current sensor contains an error attributed to the offset thereof. This results from the tolerance of the current sensor and/or aging of the current sensor and/or its peripheral circuits and devices. Due to this, in the onboard batter monitoring apparatus, the offset value of the current sensor is corrected properly.
There is known an onboard battery monitoring apparatus having a function to adjust the offset of a current sensor (for example, refer to JP-A-2004-325235). This onboard battery monitoring apparatus includes, as shown in FIG. 5, a battery 1, a current sensor 21, an ignition switch (hereinafter, referred to an “ignition switch” or “IG●SW”) 7 and a plurality of electronic control units (hereinafter, referred to as “ECEs”) 31, 11, 13, 15, and one ECU 31 of these ECUs executes the offset adjustment of the current sensor 21. The ECUs 11, 13, 15 are ECUs for controlling an engine, door locks, air conditioner, audio devices and the like.
The ECU 31 has a communication state determination unit 35D, an acquisition unit 35A for acquiring an offset adjusting timing, an output obtaining unit 35B for obtaining an output from the current sensor 21 at an offset adjusting timing so acquired and an adjustment value determination unit 35C for determining on an offset adjusting value.
In this apparatus, when the driving power supply from the battery 1 to the ECUs 11, 13, 15 is stopped with the IG●SW 7 off, the operation of switches, not shown, which connect to the ECUs 11, 13, 15 is not performed, and control signals to loads to be controlled (not shown) are not produced, resulting in a state where no control signal is transmitted and received between the ECUs 11, 13, 15 via a LAN 9.
As this occurs, only a dark current flows to the ECUs 11, 13, 15 and the controlled loads, and only a discharge current made up of the dark current and a current that is supplied to the ECU 31 flows on a power supply line 3. The amount of the discharge current is a minute value which is negligible enough in offset adjustment of the current sensor 21.
The communication state determination unit 35D detects a state where no multiplex communication is performed on the LAN 9, the acquisition unit 35A acquires a state detected by the communication determination unit 35D as an “offset adjustment condition satisfying state,” and the output obtaining unit 35B obtains an output outputted from the current sensor 21 when the acquisition unit 35A acquires the offset adjustment condition satisfying state. The adjustment value determination unit 35C determines a difference between the output of the current sensor 21 then and an output of the current sensor 21 in a standard characteristic when there is no current flowing on the power supply line 3.
In the motor vehicles produced in recent years, however, there are installed a security system which continues to its detecting operation even when the engine is stopped, entertainment equipment which can give the occupant enjoyment even while the vehicle is parked and the like, and hence, there are installed components in which consumed current (dark current) is changed and which are driven directly through the operation of switches by the occupant, even though the multiplex communication on the LAN is stopped. Due to this, even though the offset value of the current sensor is corrected by selecting a state where the multiplex communication is stopped, it is hard to carry out an accurate correction and is not possible to correct accurately a change in characteristics attributed to the tolerance and/or aging of the current sensor.